Brake system zinc-base alloy components

ABSTRACT

The present invention includes high strength, high temperature and strain rate applications of a zinc-base alloy including about 83 to 94 weight percent zinc, about 4 to about 11 percent copper and about 2 to about 4 percent aluminum. The composition may also include minor components such as magnesium and impurities. The alloy is used to construct automotive components which are subject to an instantaneous load of between 40-500 MPa. The alloy is particularly suitable for constructing components which are subject to such loads under high temperatures. In fact, components constructed from the alloy become much stronger at higher temperatures under sudden stress.

FIELD OF THE INVENTION

The invention relates to high strength, high temperature and high strainrate applications of a zinc-base alloy, and more particularly to suchapplications of a alloy including zinc, copper and aluminum for a brakesystem components.

BACKGROUND OF THE INVENTION

In a typical die casting operation, molten metal is injected at highpressure into a fixed-volume cavity defined by reusable water-cooledmetal dies. Within the cavity, the metal is molded into a desiredconfiguration and solidified to form a product casting. The metal isinjected into the cavity by a shot apparatus comprising a sleeve forreceiving a charge of the molten metal and a plunger that advanceswithin the sleeve to force the metal into the cavity. Two types of shotapparatus are known. A hot chamber apparatus comprises a shot sleeveimmersed in a bath of a molten metal. In a cold chamber apparatus, themolten charge is transferred, for example by ladle, into the shotapparatus from a remote holding furnace.

Zinc-base alloys are commonly formed by die casting, in large partbecause of a conveniently low melting point. Heretofore, zinc diecastings have exhibited a microstructure characterized by soft phases,such as the eta or alpha phases in zinc-aluminum alloys, that lackstability even at moderately high temperatures. As a result, such alloyshave had poor high temperature creep resistance that has restrictedtheir use, mainly to decorative parts.

Rashid et al., 4,990,310 discloses a creep-resistant zinc alloyincluding 4-11 percent copper, and 2-4 percent aluminum. The alloyincludes a microstructure with an intimate combination of fine epsilonand eta phases that is particularly resistant to slip. As a result, theproduct die casting from the alloy exhibits improved strength and wearresistance primarily due to the epsilon phase, but also a dramaticallyimproved creep resistance, particularly in comparison to similar zincdie castings that are substantially epsilon-free.

Commercial zinc alloys (Zamak and ZA alloys) are used mainly fordecorative applications. They are rarely used in functional/structuralapplications because their strength and/or creep properties do not meetrequirements. Instead, stronger materials like steel are used to meetspecifications. Steel parts are usually machined, whereas, zinc alloyscan be die cast to shape.

Furthermore, many automotive and nonautomotive components are requiredto withstand high forces at high strain rates. At higher temperatures(up to 150° C.) the strain rate sensitivity becomes more important sincelow melting metals such as zinc alloys usually soften at thistemperature. Thus, any increase in strength to offset this softening isan added value.

Some metals and alloys are strain rate sensitive at room temperatures,but, the magnitude of tensile strength increase is small or negligible.Stainless steel and different aluminum alloys have negligible strainrate sensitivity and the increase in tensile strength was minimal. Theincrease in strength is also very small with increasing strain rate inother types of aluminum alloys. No increase in tensile strength has beenfound in other nonferrous alloys such as copper and brass.

Different hot rolled steels, increase less than 10% in tensile strengthwith increased strain rate for the same strain rate range used in ourstudy (10⁻⁵ to 10° sec⁻¹). When iron and miled steel are tested athigher temperatures (up to 200° C.), the ultimate tensile strength doesnot increase with increasing strain rate, and they lose their strainrate sensitivity.

A variety of materials are available from which one may attempt tosuccessfully fashion components from. Many automotive components aresubject to very high loads, for example during an automobile crash. Manyautomotive components are subject to high temperatures such as thosecomponents under the hood or components which involve high temperatureapplications under dramatic loading. Conventional wisdom dictates thatsuch components are constructed from relatively expensive, heavy alloyswhich often require machining.

The present invention overcomes many of the prior art shortcomings.

SUMMARY OF THE INVENTION

The present invention includes high strength, high temperature andstrain rate applications of a zinc-base alloy including about 83 toabout 94 weight percent zinc, about 4 to about 11 percent copper andabout 2 to about 4 percent aluminum. The composition may also includeminor components such as magnesium and impurities. The alloy is used toconstruct automotive components which are subject to an instantaneousforce between 40-500 MPa. The alloy is particularly suitable forconstructing components which are subject to such loads under hightemperatures (higher than ambient temperatures). In fact, componentsconstructed from the alloy have greater relative strength at highertemperatures under sudden stress.

These and other objects, features and advantages will become apparentfrom the following brief description of the drawings, detaileddescription, and appended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a cold chamber castingzinc-aluminum-copper alloy according to the present invention;

FIG. 2 is a cross sectional view of a hot chamber die casting machinefor casting zinc-aluminum-copper alloy according to the presentinvention;

FIG. 3 is a graph of the ultimate tensile strength of an alloy used tomake components according to the present invention tested at roomtemperature with increasing strain rate;

FIG. 4 is a graph of the ultimate tensile strength of an alloy, used tomake components according to the present invention, tested at 50° C.with increasing strain rate;

FIG. 5 is a graph of the ultimate tensile strength of an alloy, used tomake components according to the present invention, tested at 100° C.with increasing strain rate;

FIG. 6 is a graph of the ultimate tensile strength of an alloy, used tomake components according to the present invention, tested at 150° C.with increasing strain rate;

FIG. 7 is a graph of the percentage increase in ultimate tensilestrength of the alloy with increasing temperature;

FIG. 8 is an illustration of an antilock brake system for a vehicleusing components made from an alloy according to the present invention;

FIG. 9 is an enlarged view with portions broken away of a motor with apinion gear shown in FIG. 8; and

FIG. 10 is an exploded view of a portion of the motor and pinion gearshown in FIG. 9.

DETAILED DESCRIPTION

Rashid et al., U.S. Pat. No. 4,990,310, the disclosure of which ishereby incorporated by reference, describes a creep resistant diecasting form from a zinc-base alloy including about 4 to about 12percent copper and about 2 to about 4 percent aluminum. That zinc,copper and aluminum alloy is now known as ACuZinc™. It has beendiscovered that the strength of a zinc-base alloy containing 4 to about12 weight percent copper and about 2 to about 4 percent aluminum,prepared by the die casting process disclosed in the '310 patent hasincreased strength when the strain rate is increased by dynamic and fastloading. The strength of the component formed from the ACuZinc™ alloy isincreased about 50 percent at room temperature, about 68% at 50° C.;about 220% at 100° C.; about 2600% at 150° C. when the loading rate wasincreased from 10⁻⁵ to 10° sec⁻¹.

The ACuZinc™ alloys which exhibit such high strength under dramaticloading and temperature, include a duplex structure having a skin offine grains of epsilon-phase (copper rich) ranging between 1-2 micronsembedded in a matrix of η (zinc rich) and Alpha phase (aluminum rich).The inner portion of the component contains larger grain sizes of theepsilon phase.

In a first example of this invention, and ACuZinc™ die casting msprepared as follows A die casting was formed of a zinc-base,copper-aluminum alloy using a conventional cold chamber die castingmachine shown schematically in FIG. 1. Machine 10 comprises a movableplaten 11 and a stationary platen 13. Die halves 12 and 14 are mountedon platens 11 and 13, respectively, and cooled by water circulatedthrough passages (not shown) therein. In the closed position shown inthe figure, die halves 12 and 14 cooperate to define a fixed-volume diecavity 16 suitably sized and shaped for producing a casting of a desiredconfiguration. At appropriate times during the casting cycle, platen 11moves relative to platen 13 to part die halves 12 and 14 along a planeindicated by line 18 for ejection of a product casting. Machine 10 alsoincludes a shot apparatus 20 comprising a generally cylindrical shotsleeve 22 that communicates with cavity 16. Sleeve 22 includes an inlet24 for admitting a molten metal charge 26 poured, for example, from asuitable ladle 28. A hydraulically driven shot plunger 30 is slidablyreceived in sleeve 22 and advances toward the die sections for forcingmetal from sleeve 22 into cavity 16.

In accordance with a preferred embodiment of this invention, charge 26was composed of an alloy comprising 10.0 weight percent copper, 3.6weight percent aluminum, 0.03 weight percent magnesium and the balancezinc and impurities. The charge was poured at a temperature of about532° C. into shot sleeve 22 through port 24. Slot plunger 30 wasadvanced to inject the charge into casting cavity 16. The cavity surfacetemperature was about 140° C. After filling the die cavity, the shotplunger continued to apply a load of 1340 kilograms for about 12seconds. Within the die cavity, the metal cooled and solidified,whereafter the die sections were parted to eject a product casting.

In a second embodiment, zinc die castings of this invention weremanufactured using a hot chamber die casting machine 50 shownschematically in FIG. 4. Machine 50 comprises water-cooled die halves 52and 54 mounted on a stationary platen 53 and a movable platen 55,respectively, adapted for moving die halves between a closed positionshown in FIG. 4 wherein the die halves cooperate to form a castingcavity 56 and an open position wherein the die halves are parted along aplane indicated by line 58 for ejection of a product casting. Inaccordance with common hot chamber die casting process, die castingmachine 50 comprises a shot apparatus 60 formed of a goose neck sleeve62 partially submerged in a molten metal bath 64 contained in meltingpot 63. Shot apparatus 60 further comprises hydraulically driven plunger68 slidably received in goose neck 62. When plunger 68 is in a retractedposition shown in the figure, a charge of molten metal from bath 64fills goose neck 62 through an inlet port 66. For casting, plunger 68 isdriven downwardly to force molten metal through sleeve 62 into diecavity 56.

In accordance with this invention, a hot chamber die casting was formedof an alloy containing 5.0 weight percent copper, 3.0 weight percentaluminum, 0.035 weight percent magnesium and the balance substantiallyzinc. The temperature of the charge was about 490° C. The casting cavitysurface temperature was about 150° C. During injection, the melt wassubjected to a pressing load of 62 kiloPascals. Other materials with3.5-12.0 weight copper, 1-8 weight Al and 0.01-0.06 Mg of compositionwill have similar effect of strain rate on loading at the above testedcomposition.

Chemical composition of the test specimens used in this investigationwas as described above. Specimens tested were 25.4 mm long×5.33 mmdiameter in the gauge section, and were used in the as-diecastcondition, with the as-cast surface intact. Tests were performed usingan Instron universal testing machine which has the capability ofmaintaining constant cross-head speed. Load-elongation data wererecorded automatically during the tests. The specimens were loaded tofailure and ultimate tensile strength calculated. Tests were conductedat four different temperatures (room temperature, 50° C., 100° C., and150° C.) using a temperature controlled chamber to insure constanttemperature during the test. Cross-head speeds used were 0.02, 0.2, 2,20, 200 and 500 mm/min, which provided strain rates ranging from1.3×10⁻⁵ to 3.2×10⁻¹ sec⁻¹.

FIG. 3 shows the variation of ultimate tensile strength (UTS) of thisalloy at room temperature (20° C.) with increasing strain rate. It wasfound that when the strain rate is increased from 10⁻⁵ to 10° sec⁻¹, theUTS increased from 280 to 440 MPa (57%).

The percentage increase in ultimate tensile strength with increasingstrain rate was found to be much higher when the specimens were testedat higher temperatures. At 50° C. the UTS increased from 250 to 420 MPa(68%) for the same increase in strain rate (FIG. 4). The percentageincrease was 220% at 100° C. (FIG. 5), and 2600% at 150° C. (FIG. 6).

The above data is replotted in FIG. 7 to show the percentage increase inUTS with temperature when the strain rate increases from 10⁻⁵ to 10°sec⁻¹. It is clearly shown that the strain rate sensitivity increasesdramatically above 80° C.

A typical microstructure of the alloy has a duplex structure consistingof an outer skin of fine grains of ε-phase (copper rich) ranging between1-2 microns embedded in a matrix of η (zinc rich) and α phases (aluminumrich). The microstructure is much coarser inside the specimen.

In this investigation, we discovered that ACuZinc™ alloys willstrengthen when the strain rate increases between 10⁻⁵ to 10° sec⁻¹, andthis increase is greater at higher temperatures. This behavior isunexpected and has not been reported before. Alloys such as aluminum,copper, stainless steel are not strain rate sensitive when tested atroom temperature. The strain rate sensitivity of pure iron and steelalloys is much smaller than that found in ACuZinc™ alloys at roomtemperature. Iron and steels lose their strain rate sensitivity whentested at higher temperatures (reported up to 200° C.).

With this discovery ACuZinc™ die cast alloys can be used with confidenceat higher temperatures for components in fast loading/high temperatureapplications. The unexpected increase in strength with strain rateduring high temperature testing provides increased potential use ofthese parts in many applications such as for under-hood automotiveapplications use, such parts as connectors, brackets, support, andengine mounts, which can be subjected to a combination of high strainrates and high temperature conditions as during an impact in acollision. It may also be applicable in other systems such as the airbag system and components and attachments used to secure the seat beltassembly. Seat actuators, and components in the steering columns, gears,racks, supports, and housing are other potential components, whereincrease strength at high rate deformation is important.

Data:

In Table 1 the Ultimate Tensile Strength of the alloy tested is comparedto commercial zinc base alloys (Zamak 3 and ZA 8 alloys) tested at twodifferent temperatures and strain rate of 1.312×10⁻³ sec⁻¹. The resultsshow that the ultimate tensile strength at 50° C. for ACuZinc™ was thesame as that measured at room temperature (20° C.). 351 MPa at 50° C.(100° F.) compared to 347 MPa at room temperature. However, the UltimateTensile Strength of commercial Zamak 3 alloy was 253 MPa at 20° C. but227 MPa when is tested at 50° C.; for ZA 8 alloy it was 336 MPa at 20°C. vs. 283 MPa when tested at 50° C.

                  TABLE 1    ______________________________________               Alloy Tested Zamak 3   ZA 8    Temperature               UTS, MPa     UTS, MPa  UTS, MPa    ______________________________________    20° C.               347          253       336    50° C.               351          227       283    ______________________________________

Components constructed from the ACuZinc™ alloy provide resistance todeformability and damage when the components are under mechanicalloadings at high strain rates or when they are impacted as in a crash orcollision situation. Even though ACuZinc™ is a zinc-base alloy, it hassurprisingly been discovered that high load bearing componentsmanufactured from ACuZinc™, that is, components subjected to loadingsbetween 40-500 MPa, exhibited a dramatic improvement in strength.Examples of high load components include seat belt assembly components,actuators, gears, seat actuators, seat racks, racks, motor mounts,electronic housings, and other components which must remain functionalduring an automotive collision.

Nonautomotive components, subject to dramatic load increases and at hightemperatures may be made from the ACuZinc™ alloy. For example,components for hand tools such as drills, may be constructed fromACuZinc™ alloys. Such materials are often subject to high loads andtemperatures. For example, when a saw or drill having ACuZinc™components hits a nail or other material, the ACuZinc™ componentsactually become stronger under such high loadings and temperatures.

Many of the components in an automobile, such as those under the hood inthe engine compartment, are subject to high temperatures up to 150° C.In an automobile collision, it is important for these components remainfunctional so that the automobile can continue to operate or can bedriven away from the scene of an accident. Accordingly, components suchas hose couplings, battery connectors and extensions, engine bracketsand mounts may be constructed from ACuZinc™. Particularly important arehousings for electronic components, such as the automotive computermodule housing which may be constructed out of ACuZinc™. Upon impact ina collision, an ACuZinc™ electronic housing is much more likely tosurvive the impact than other materials. If the computer module isdamaged, the car will not be driveable.

Nonautomatic electronic components may be housed by structures formedfrom ACuZinc™ material when the housings are subjected to high loads andtemperatures. For example, an electronic housing for a cluster bomb issubject to high loadings and high temperatures during the firstexplosion of the bomb, which sends smaller bombs in a variety ofdirections. The timers within the electronic components of each of thesmaller bombs must survive the initial explosion in order for thesmaller bombs to function properly. Electronic housings made fromACuZinc™ provide an improved housing with greater strength duringdramatic loads and high temperatures.

One preferred embodiment of the invention is a vehicle wheel anti-lockbraking system 107 which includes a master cylinder 112 for supplyingpressurized fluid. Connected on the wheel 114 and schematically shown,is a fluid activated wheel brake cylinder 116 (hereinafter referred toas a wheel brake) which receives pressurized fluid from the mastercylinder for restraining rotational movement of the wheel 114. The wheelbrake 116 may be utilized in a conventional drum or disc type vehiclebrake.

An ABS electronic controller 118 is also provided. A sensor 120 in thevehicle wheel brake 116, determines the wheel 114 rotational speed and asensor (not shown) determines whether or not the brake pedal 122 of thevehicle is activated. Both sensors feed information to the ABScontroller 118. The ABS controller 118 will be cognizant of therotational condition of the wheel and will provide an appropriate signalin response thereto. The signal will place the brake system in an ABSmode of operation if the condition of the wheel 114 is within presetparameters.

A normally open solenoid valve 124, when activated to a closed positionin response to a signal given by the controller 118, functions as anisolation valve to prevent fluid communication between the mastercylinder 112 and the wheel brake 116. An actuator 128 is provided havingan actuator frame 130 with a longitudinal bore 132. An actuator can beprovided for each wheel brake of the vehicle or if desired, a pluralityof wheel brakes can be connected to a single actuator. The longitudinalbore 132 has a first fluid connection 142 allowing fluid communicationwith the wheel brake 116 and the longitudinal bore 132 also has fluidcommunication with the master cylinder 112 when the solenoid valve 124is not activated to the closed position via passage 140. Additionally,as shown, the longitudinal bore has a second or alternative fluidcommunicative path with the master cylinder 112. As shown, the bore 132is midstream of the solenoid valve 124 and passages 142. Fluid flowpasses over a transverse slot (not shown) of a piston 144. However, thesolenoid valve 124 could directly tie into the wheel brake 116 andpassage 142 could "T" into that line. The alternative path 134 has acheck valve 138 whose function will be described later. The check valve138 allows delivery of fluid back to the master cylinder 112 wheneverthe wheel brake 116 has a pressure greater than that in the mastercylinder 116. Therefore, the braking system is sensitive to an operatorrelieving the brake by removing his or her foot therefrom without anyneeded input from the controller.

The piston 144 is slidably and sealably mounted within the longitudinalbore 132. Movement of the piston 144 provides a variable control volumein communication with the wheel brake 116, thereby modulating thepressure therein. A nut 146 operatively associated with piston 144 isconnected with the piston 144 and the nut 146 is slidably mounted withinthe longitudinal bore 132 in a non-rotative fashion.

A power screw 148 projects into the nut and is threadably engagedtherewith in an efficient manner. The power screw has a fixed rotationalaxis with respect to the actuator frame 130. Powering the power screw isa reversible DC motor 150 which is responsive to the signals given to itby the controller 118. In the position shown, for normal brakingoperation, the piston 144 is held at the extreme up position and must beheld within a tolerance of 3/100 of an inch to maintain the check valve138 in the open position via the rod 152 (tolerance shown in FIG. 1greatly enlarged for purposes of illustration).

Power screw 148 is connected to gear train 180 which is in turnconnected also with motor 150. The power screw is mounted by bearingsand has a large gear 182 connected at one end. The large gear 182 mesheswith a smaller pinion gear 186. The pinion gear 186 axially floats on arotor shaft 180 of the motor and is held on by spring clip 190. Themotor pinion gear 186 has a drive dog 192 downwardly extending from oneface for operatively engaging an expansion spring 194 having a tang 196at one end. A drive member 198 is connected to the motor shaft and hastwo upwardly extending drive dogs 200, 202 in opposed positions foroperatively engaging a second tang 204 on the other end of the spring194. A sleeve 206 surrounds the motor shaft 180. The ABS electroniccontroller is used to selectively supply power to the motor under theconditions described above. The expansion spring applies a brakingaction to the motor shaft at the moment the power to the motor ceases.The spring does this by gripping the shaft in manner similar to atransmission's overriding clutch. A detailed description of theoperation of such mechanisms which are known to those skilled in the artis disclosed in U.S. Pat. No. 5,246,282 which is hereby incorporated byreference. According to the present invention, the motor pinion gear 186may be made from the ACuZinc™ alloy. The motor pinion gear 186 undergoeshigh stress and high temperature when the ABS system is in operation dueto the sudden load applied by the motor and the heat generated from themotor pack and the work done by the motor pinion on the large gear. Whenthe ABS system is activated, a stress of 113 newton meters/second (160oz in over 10.5 milliseconds) is applied to the newton pinion gear 186.The large gear 182, expansion spring 194 and motor drive member 198 mayalso be made from the ACuZinc™ alloy according to the present invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An anti-lock brakingsystem (ABS) for a wheel of an automotive vehicle comprising:a mastercylinder for supplying pressurized fluid; a sensor to determine arotational speed of a wheel; a wheel brake receiving pressurized fluidfrom said master cylinder means and for restraining rotational movementof said wheel; an ABS controller cognizant of the rotational conditionof said wheel via the sensor and providing a signal when the rotationalcondition of said wheel is within present parameters; an actuator framehaving a bore with fluid communication with said wheel brake means; apiston slidably sealably mounted with said bore for providing a variablecontrol volume in communication with said wheel brake and therebymodulating the pressure therein; a nut operatively associated with saidpiston and slidably mounted within said bore in a non-rotative fashion;a power screw projecting into said nut and threadedly engaged within ina low friction backdriveable manner, said power screw having a fixedrotational axis with respect to said actuator frame; a drive gearconnected to said power screw; reversible motor for powering said powerscrew, said motor being responsive to signals given by said controller;a pinion gear operatively connected to said reversible motor andengaging said drive gear to turn said power screw; and wherein saidpinion gear comprises an alloy consisting essentially of, by weight,between about 4 and 12 percent copper, 2 and 4 percent aluminum, and thebalance zinc and impurities and having fine epsilon and eta grainsdispersed in a ternary eutectic matrix.
 2. An anti-lock braking systemsas set forth in claim 1 further comprising a drive dog downwardlyextending from one face of the pinion gear for engagement with anexpansion spring having a tang at one end;a motor shaft operativelyconnected to the motor and a drive member connected to the motor shafthaving two upwardly extending drive dogs in opposed positions foroperatively engaging a second tang on the other end of the spring.
 3. Ananti-lock braking system as set forth in claim 2 wherein said drivemember comprises said alloy.
 4. An anti-lock braking system as set forthin claim 2 wherein said spring comprises said alloy.